Measuring Population HealthMeasuring Population Health

Learning ObjectivesLearning Objectives

To explain the difference between To explain the difference between numerators and denominators, and their numerators and denominators, and their importance in population measurementimportance in population measurement

To distinguish between different To distinguish between different measures of health or disease frequency measures of health or disease frequency in populationsin populations

RateRate

In epidemiology, demography and vital statistics, a In epidemiology, demography and vital statistics, a rate is an expression of the frequency with which rate is an expression of the frequency with which an event occurs in a defined population in a an event occurs in a defined population in a specified period of time. specified period of time.

The use of rates rather than raw numbers is The use of rates rather than raw numbers is essential for comparison of experience between essential for comparison of experience between populations at different times, different places or populations at different times, different places or among different classes of persons. among different classes of persons.

RateRate

n

periodtheduringpopulationAverage

periodspecifiedainEventsofNumberRate 10

(Numerator)

(Denominator)

The components of a rate:The components of a rate:NumeratorNumeratorDenominator Denominator Specified time in which the events occurSpecified time in which the events occura multiplier (a power of 10) that converts the rate from an a multiplier (a power of 10) that converts the rate from an awkward fraction or decimal to a whole numberawkward fraction or decimal to a whole number

Incidence RateIncidence Rate

Number of Number of newnew cases of disease over a cases of disease over a period of timeperiod of time

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Population at RiskPopulation at Risk

Incidence RatesIncidence Rates

Need:Need:– A period of observationA period of observation– Numerator (number of new cases)Numerator (number of new cases)– Times of disease onset (diagnosis)Times of disease onset (diagnosis)– Denominator (Population at risk)Denominator (Population at risk)

Measures of MorbidityMeasures of Morbidity

Incidence measures the rate at which Incidence measures the rate at which people without the disease develop the people without the disease develop the disease during a specified period of timedisease during a specified period of time

Used to study disease aetiology (risk)Used to study disease aetiology (risk)

Low incidence constantly present

High incidence at set time, above endemic levels

Global epidemics

Infection in a Population

Prevalence RatePrevalence Rate

Number of Number of existingexisting cases of disease at a point cases of disease at a point in timein time

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Total PopulationTotal Population

PrevalencePrevalence

Prevalence measures the number of people Prevalence measures the number of people in a population who have the disease at a in a population who have the disease at a given point in timegiven point in time

Depends on Incidence and durationDepends on Incidence and duration P ~ I X DP ~ I X D Used to measure disease burdenUsed to measure disease burden

Prevalence RatesPrevalence Rates

Need:Need:– Definite point in timeDefinite point in time– Numerator (number of existing cases)Numerator (number of existing cases)– Time of disease onsetTime of disease onset– Denominator (size of population)Denominator (size of population)

Factors that Influence PrevalenceFactors that Influence Prevalence

Change in Incidence ( )Change in Incidence ( ) Longer duration of the diseaseLonger duration of the disease Prolongation of life of patients without cureProlongation of life of patients without cure Change in case fatality rateChange in case fatality rate Selective migrationSelective migration

– In-migration of cases, out-migration of healthy peopleIn-migration of cases, out-migration of healthy people In-migration of susceptible peopleIn-migration of susceptible people Improved diagnostic facilitiesImproved diagnostic facilities Primary and Secondary Prevention strategiesPrimary and Secondary Prevention strategies

Relationship Between Incidence and Relationship Between Incidence and PrevalencePrevalence

Prevalence depends on incidence Higher incidence leads to higher prevalence

if duration of cases does not change. Limitation of the bathtub analogy –flow rate

needs to be expressed relative to the size of the source

Prevalence = Incidence x Duration of disease

The Bathtub Analogy

Secular trend refers to a change in the prevalence of infection over years. This relates to better living conditions, better hygiene, and vaccination. An example of a secular trend is the decrease in tuberculosis (TB) in UK.

Seasonal trend refers to changes in the prevalence of infection occurring over the year, e.g., flu outbreaks - the reason the seasonality is unclear but changes of temperature, crowding and humidity may play a role.

Trends

References:References: Beaglehole, R., Bonita, R., and KjellstrBeaglehole, R., Bonita, R., and Kjellström, T. öm, T.

(1993) (1993) Basic EpidemiologyBasic Epidemiology. 2. 2ndnd ed. World Health ed. World Health Organisation 2006.Organisation 2006.

Farmer, R., Miller, D., Lawrenson, R. (2000)Lecture Farmer, R., Miller, D., Lawrenson, R. (2000)Lecture Notes on Epidemiology and Public Health Medicine, Notes on Epidemiology and Public Health Medicine, 44thth Edition, Blackwell Science. Edition, Blackwell Science.

HOUSTON, J.C., JOINER, C.L., and TROUNCE, HOUSTON, J.C., JOINER, C.L., and TROUNCE, J.R. (1979) J.R. (1979) A short textbook of medicine.A short textbook of medicine. 6thed. 6thed. Sevenoaks: Hodder and Stoughton. Sevenoaks: Hodder and Stoughton.

Rothman, KJ., Greenland, S. (1998) Modern Rothman, KJ., Greenland, S. (1998) Modern Epidemiology, 2Epidemiology, 2ndnd Edition, Lippincott Williams and Edition, Lippincott Williams and Wilkins.Wilkins.